[DefaultValue("")]
public string GroupColumn { get; set; }

<DefaultValue("")>
Public Property GroupColumn As String

<dx:ASPxGridView ID="grid" runat="server" AutoGenerateColumns="False" KeyFieldName="ProductID">
    <Columns>
        <dx:GridViewDataTextColumn FieldName="ProductID" VisibleIndex="0" />
        <dx:GridViewDataTextColumn FieldName="CategoryName" VisibleIndex="1" GroupIndex="0" />
        <dx:GridViewDataTextColumn FieldName="ProductName" VisibleIndex="2" />
    </Columns>
    <GroupSummary>
        <dx:ASPxSummaryItem FieldName="ProductName" SummaryType="Count" />
    </GroupSummary>
</dx:ASPxGridView>

protected void Page_Load(object sender, EventArgs e) {
    grid.GroupSummarySortInfo.Clear();
    ASPxGroupSummarySortInfo sortInfo = new ASPxGroupSummarySortInfo();
    sortInfo.SortOrder = ColumnSortOrder.Descending;
    sortInfo.SummaryItem = grid.GroupSummary["ProductName", SummaryItemType.Count];
    sortInfo.GroupColumn = "CategoryName";
    grid.GroupSummarySortInfo.AddRange(sortInfo);
}

Protected Sub Page_Load(ByVal sender As Object, ByVal e As EventArgs)
    grid.GroupSummarySortInfo.Clear()
    Dim sortInfo As ASPxGroupSummarySortInfo = New ASPxGroupSummarySortInfo()
    sortInfo.SortOrder = ColumnSortOrder.Descending
    sortInfo.SummaryItem = grid.GroupSummary("ProductName", SummaryItemType.Count)
    sortInfo.GroupColumn = "CategoryName"
    grid.GroupSummarySortInfo.AddRange(sortInfo)
End Sub